What becomes of all the sediments?

It used to be widely thought that sediment of the ocean floor and that at active continental margins or ahead of volcanic arcs were scraped off subducting lithosphere and simply added to continental growth. If that didn’t happen, then perhaps continents could be recycled by a combination of erosion and tectonics? Geochemists know better now, for a variety of compositional anomalies in volcanic rocks do suggest a measure of recycling of subducted lithosphere, and it is becoming clear that part of the oddity has a sedimentary source. “Which one?” is the question.

Hafnium and neodymium isotopes have become choice tracers of whether basaltic magmas formed from pristine mantle, that depleted by previously sourcing magma or some kind of mixture with recycled materials. . Catherine Chauvel and colleagues from the University of Grenoble have pondered on the sizeable amount of Hf and Nd isotopic data that has emerged from a couple of decades of fancy mass spectrometry of ocean-island and mid-ocean-ridge basalts, and a variety of sediments (Chauvel, C. et al. 2008. Role of recycled oceanic basalt and sediment in generating the Hf-Nd mantle array. Nature Geoscience, v. 1, p. 64-67). By modelling how various reasonable mixtures of isotopes of the two elements might fit the simple Hf-Nd relationship for the source mantle of all oceanic basalts they discovered that it couldn’t be derived from just the crystalline oceanic lithosphere, but must involve a substantial contribution from subducted sediments. Moreover, they seem to have demonstrated that much of the mantle involved in producing ocean-island, hot-spot basalts is a product of this recycling – both oceanic crust and its sedimentary cover get down to the levels where the mantle involved in hot-spot melting originates. Although there is a good probability of separation of sediment and crystalline components of subducted slabs according to density, it seems from the modelling that some sediment does get down to profound levels.

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